The ride, which normally costs $3,500, was provided to Hawking at no charge.

The 65-year-old scientist, who holds a research post at the University of Cambridge, said he wanted to make the flight to build awareness for the importance of space exploration.

The plane flew eight parabolic manoeuvres

"I believe that life on Earth is at an ever-increasing risk of being wiped out by a disaster such as sudden global warming, nuclear war, a genetically engineered virus or other dangers," Hawking said before his flight.

"I believe the human race has no future if it doesn't go into space."

Confined to a wheelchair due to his degenerative and irreversible motor neurone disease, Hawking had hoped to be able to complete one parabolic manoeuvre, which generates about 25 seconds of weightlessness.

Upon reaching an altitude of about 24,000ft above ground, the plane, a specially modified Boeing 727 jet, hikes steeply towards the sky, then arches over to plunge downward. The descent temporarily lifts the bonds of gravity.

In the end, Hawking and the 35 other people aboard the plane ended up making eight parabolic manoeuvres.

"He didn't want to come back," Diamandis said.

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After the flight, Hawking used his computer, which he controls by twitching a cheek muscle, and voice synthesizer to tell reporters that the zero-gravity part of his flight was wonderful and that he had no problems handling the extra forces generated by the climb into the sky.

"I could have gone on and on," he said.

The flight, which took off and landed at the Kennedy Space Center in Florida, also was intended to test if Hawking has the "right stuff" for a suborbital spaceflight.

The professor has a reservation for a ride on Virgin Galactic, which is expected to begin commercial suborbital spaceflights in 2009.